The present invention relates to a method for contactless treatment of a substrate such as a semiconductor wafer, comprising enclosing the wafer in an apparatus and applying two gas streams, in opposing directions, from first and second side sections located opposite on another, to the two opposing planar sides or surfaces of the wafers.
A method of this general type is disclosed in Netherlands Laid Open Application 8402410, in which furthermore, reference is made to Netherlands laid Open Applications 8103979, 8200753 and 8203318.
From these publications it is known to position a wafer such that it is floating between the two side sections. If the gas flow is suitably chosen, it has been found that a highly accurate definition of the position of the wafer with respect to the side sections is possible and this position is relatively fixed, that is to say little variation occurs in the position of the wafer with respect to the side sections. In the patent publications concerned it is described that the wafer is subjected to a wet treatment and is then possibly dried. For the purposes of drying, the gas which holds the wafer in place is heated to about 100xc2x0 C. and is moved over the surface of the wafer, as a result of which the moisture present is automatically removed.
Heating to much higher temperatures in the range of 200xc2x0-1200xc2x0 C. is frequently necessary when treating semiconductor substrates. Heating can involve annealing or raising the temperature to make deposition or other processes possible. In the prior art, wafers are to this end placed in furnaces and then heated. Although this method is adequate, it has at least two disadvantages. Firstly, a method of this type is usually not completely contactless, that is to say certain points of the wafer are supported. Secondly, it takes relatively long time to heat a wafer. This is due not so much to the thermal capacity of the wafer itself as to the relatively slow heat transfer between the furnace and the wafers and to the fact that in order to achieve a controlled, deformation free heating of the wafers, the wafers need to be heated inside the furnace together with the furnace itself.
In order to solve this problem single wafer systems have been disclosed with which rapid heating was achieved with the aid of high power lamps (50-80 Kw). Such a method is particularly expensive and difficult to control.
U.S. Pat. No. 4,622,918 discloses an apparatus wherein a wafer is fed through between a number of columns located some distance apart. Sets of columns located opposite one another, between which the wafer moves, are likewise some distance apart. In the gap between the sets of columns, heating is effected by means of a lamp some distance away. This apparatus has the drawback that due to the presence of many metal parts with complicated constructions in close proximity to the wafer, only heating to limited temperatures is possible. Furthermore, in this apparatus a wafer is supported by a plurality of columns of air streams with gaps in between the columns. Due to the succession of columns, where the wafer is supported, and gaps between the columns where the wafer is exposed to the heat radiation of the lamps, both the support of the wafer and the heating are not homogeneous.
It has been found that only limited heating can take place effectively by heating the gases, as is described in the above-mentioned Netherlands applications.
The aim of the present invention is to provide a method with which contactless heating of semiconductor substrates to relatively high temperature within a relatively short time is possible.
This aim is achieved by enclosing a wafer between relatively massive side sections of an apparatus and applying gas streams to opposite planar sides of a wafer. Preferably the side sections include plates having a minimum thickness of about 10 mm with the spacing between each of the side sections and the wafer to be at most about 1.0 mm. Further, at least one of the side sections is heated to a temperature higher than 200xc2x0 C.
Surprisingly, it has been found that if the spacing between the side sections, or between side section and wafer, is set to be relatively small, particularly rapid heat transfer can take place. It is possible to achieve heating to far above 1000xc2x0 C. within a few seconds. Because with this arrangement the wafer, in principle, does not have to be supported, but is held accurately and definitively in its place by the gas streams, the wafer will also not be subjected to stresses generated by local temperature differences and distortion will be prevented as far as possible. Incidentally, it is pointed out that if a slight degree of distortion does take place, the stabilizing effect of the gas streams in opposing directions is such that the wafer is pressed straight in a xe2x80x98gentlexe2x80x99 manner without damage.
Therefore, it is now possible to keep the apparatus at the process temperature and load the wafer while the side sections are at process temperature without damage to the wafer. As a consequence, a particularly small peak power is needed to achieve such rapid heating of wafer because the energy required to heat the wafer is withdrawn from the side sections. It will be understood that the method described above is outstandingly suitable for processes in which wafers are treated one-by-one (xe2x80x98single wafer processingxe2x80x99). However, it is also possible to treat large numbers of wafers one after the other or parallel to one another in the manner described above.
Introduction of the wafer into the enclosing apparatus described above can be effected in any way known from the prior art. A particularly simple method is that in which the side sections can be moved apart. The wafer can be placed between the side sections when the latter have been apart. Supporting means can optionally be present to fix the wafer in such a position. The side sections then move towards one another and the function of the supporting means can be taken over by the gas stream moving out of the side sections concerned. As a result, the wafer moves away from the supporting means.
Apart from heating the semiconductor substrate in this way it is also possible to carry out treatments on the substrate, such as oxidation, itching or the deposition of layers. To this end it is possible to mix a gaseous medium with the gas which holds the wafer in its place. Of course, it is also possible to position the wafer using process gas only. This is in contrast to what is described and suggested in the above-mentioned Netherlands Applications, where only wet treatment of the related substrate takes place. This process gas can be supplied, uniformly distributed, from one of the side sections, such that a uniform distribution over the related wafer surface takes place.
One of the problems encountered in the prior art when supplying process gas at elevated temperature, and more particularly when depositing layers, is that the apparatus used to supply the process gas becomes contaminated by deposition of the material concerned from the process gas. This means that apparatuses of this type have to be cleaned regularly and that major problems arise with regard to clogging.
With the method according to the invention it is possible to prevent these problems. This is because, according to a further embodiment of this method, a temperature difference is applied over the wafer. One of the side sections is heated to a relatively high temperature, whilst the other of the side sections is heated to a relatively low temperature. It has been found that, as a result of the thermal behavior of the enclosing apparatus, the wafer will assume a temperature which is dependent on the position of the wafer with respect to the two heated side sections. If the two side sections are equidistant from the wafer and the same gas is present on both sides, the temperature will fairly accurately be the average of the values of the temperatures of each of the side sections.
If, by controlling one or both gas streams, the wafer is not located centrally between the two side sections, the temperature will change correspondingly.
If different types of gas are used, that is to say gases having different thermal conduction properties, a change in temperature will likewise take place. For example, when argon is used on one side and hydrogen is used on the other side it has been found that transfer between the relevant side section and the wafer is ten times better on the side where hydrogen is supplied.
Consequently, by means of a suitable choice of the temperatures concerned, it is possible to provide the side section from which the process gas is emitted with a temperature such that no deposition takes place on such side section, whilst the wafer is at a temperature which is so much higher that deposition does take place on such wafer.
It has been found that the rate of deposition of, for example, polysilicon from silane on a substrate is lower by a factor of 350 at 700 K and a partial pressure of 0.4 torr than at 900 K. This means that by controlling the temperature, deposition is negligible on the side section from which the process gas is supplied and which is at low temperature.
With this arrangement it is possible, in the starting position, to place the wafer with the xe2x80x98device sidexe2x80x99 of the wafer towards the side section which is at the lowest temperature, through which side section the process gas is subsequently supplied. As a result of supplying the reactive gases, the wafer is moved towards the side section at the higher temperature and, on assuming the higher temperature, deposition accordingly, takes place. The reverse set-up is also possible. That is to say, the side section from which the gas emanates is at a higher temperature than the opposite side section. In this case, the xe2x80x98device sidexe2x80x99 of the wafer faces the side section which is at the lower temperature and the Bernoulli principle can be used by allowing the correct gas stream to flow against the top of the wafer. With this arrangement a reduced pressure is created beneath the wafer, which reduced pressure ensures that the wafer will float (in a stable manner) beneath the top side section. The hot (bottom) side section is then raised until the process situation is achieved.
It has been found that appreciable temperature differences between the related side section and the wafer are possible using the construction described above. A value of at least 150xc2x0 C. and more particularly 200xc2x0 C. may be mentioned by way of example.
With the method according to the invention, these values can be set very accurately. After all, it has been found that these values are mainly dependent on the position of the wafer in the enclosing apparatus. As already indicated above, the position of the wafer in the tunnel-like apparatus is accurately related to the quantity and type of gas supplied from the related side sections.
The invention also relates to an apparatus with which the above method can be carried out in all its alternative embodiments. In this apparatus at least one of the side sections is provided with heating means for heating said section(s) to above 250xc2x0 C. It has been found that relatively small peak power is needed to achieve relatively fast heating of a wafer. It is in particular the high thermal capacity of the side section concerned, which is of importance for the stability of the process.
The related side section can be provided with a number of spaced gas feed channels in order to provide uniform metering of the gas and more particularly process gas.
In a simple embodiment which is particularly suitable for deposition purposes, a very large number of injection points must be present. A construction of this type can, for example, be achieved by providing porous plates.
The invention will be explained below with reference to an illustrative embodiment shown in the drawing. In the drawing: